1. Description of the Conventional Art
In the recent information age, a display device is becoming more and more important as a visual information transmission media. The display device has to satisfy low power consumption requirements, be thin and light, produce high image quality, etc.
A liquid crystal display (LCD) device displays an image by using optical anisotropy of a liquid crystal material. The LCD has better viewing characteristics and less power consumption and heating than the conventional cathode-ray tube (CRT). These characteristics promote the LCD device as the next generation display device.
A liquid crystal material used in the LCD device is not a spontaneous light emitting material, but material receptive of light for displaying an image by modulating an amount of external light. Thus, in the conventional LCD device, an additional light source for irradiating light to a LCD panel, that is, a backlight unit is required.
Generally, an LCD device contains a LCD panel with a thin film transistor array (TFT) substrate and a color filter (CF) substrate that face each other are attached to each other to have a certain cell gap. A driving unit is provided for driving the LCD panel. A back-light unit is provided at a rear surface of the LCD panel for supplying light to a liquid crystal layer of the LCD panel.
On the TFT substrate of the LCD panel, a plurality of gate lines arranged in a horizontal direction at a regular interval are perpendicular to a plurality of data lines arranged in a vertical direction at a regular interval. The intersection region formed by the crossing gate lines and the data lines is defined as a pixel.
The pixel is provided with a switching device, such as a thin film transistor, and a pixel electrode. The switching device is electrically connected to the data line and the gate line to receive image information through the data line and to apply the image information to a pixel electrode while maintaining a turn-on state by a gate voltage applied through the gate line. The pixel electrode forms an electric field with a common electrode provided at the CF substrate thereby changing the orientation of the liquid crystal molecules.
The CF substrate is provided with red (R), green (G), and blue (B) color filters for separating light supplied from the back-light unit into R, G, and B light and for displaying various color images by mixing the R, G, and B light. A black matrix for preventing light leakage is formed at outer peripheries of the R, G, and B color filters. A common electrode is formed at whole surface of the CF substrate, and applies an electric field to the liquid crystal layer with the pixel electrode thereby to change an arrangement form of the liquid crystal molecules. This permits the common electrode to control the transmittance of light supplied from the back-light unit and display an image of the LCD panel.
The driving unit consists of a data driving unit and a gate driving unit. When the gate driving unit sequentially applies a gate voltage to each gate line, the switching devices electrically connected to the gate line to which the gate voltage is applied are turned on and the turned on switching devices receive image information from the data lines. At this time, the image information is applied to the pixel through the switching device, and is applied to the pixel electrode provided in the pixel.
The LCD device uses a holding method. When the gate driving unit sequentially applies a gate high voltage to each gate line, image information applied to the pixel through the data line is applied to the pixel electrode thereby to be charged at a storage capacitor. The storage capacitor maintains the state of the liquid crystal molecules for one frame even when a gate high voltage is transferred into a gate low voltage thereby to maintain a brightness of the pixels.
The LCD device can be driven by an external or internal voltage change. When devices inside the LCD device are destroyed by an electrostatic discharge or an over-voltage, the image quality is degraded. In order to reduce the damage due to a sudden high voltage, an electrostatic preventing unit is provided in the LCD device.
FIG. 1 is a drawing schematically showing a general LCD device.
As shown in FIG. 1 the LCD device consists of a LCD panel 1 with a TFT substrate (not shown) and a CF substrate (not shown) facing each other and attached to each other to have a certain cell gap. A plurality of data lines 15 are arranged on the LCD panel 1 in a vertical direction and a plurality of gate lines 16 are arranged on the LCD panel 1 in a horizontal direction. A data driving unit 5 supplies image information to the data lines 15 and a gate driving unit 6 supplies control signals and driving voltages to the gate lines 16. A common voltage line 32 formed on the thin film transistor array substrate applies a common voltage (VCOM) to a common electrode of the color filter substrate. An electrostatic preventing unit 10 is connected to the gate lines 16 and the data lines 15. The electrostatic preventing unit 10 disperses the current.
The intersection regions of the data lines 15 and the gate lines 16 are defined as pixels P1. The pixels P1 are arranged on the TFT substrate in a matrix thereby to constitute an image display unit 20 for substantially displaying an image.
The LCD device is provided with a timing control unit and a power supplying unit. Image information and the control signals outputted from the timing control unit, and the driving voltages outputted from the power supplying unit are supplied to the gate driving unit 6 through the data driving unit 5.
As shown, the data driving unit 5 and the gate driving unit 6 are electrically connected to each other through a connecting line 34. The data driving unit 5 applies a gate low voltage VGL to the gate driving unit 6 through the connecting line 34. Although not shown, a plurality of lines are formed in order to connect the data driving unit 5 and the gate driving unit 6. Through the plurality of lines, the gate driving unit 6 receives a gate high voltage VGH and a common voltage VCOM. The common voltage VCOM is formed along an outer periphery of the thin film transistor array substrate, and is applied to the gate driving unit 6 through the common voltage line 32 connected to the data driving unit 5 and the gate driving unit 6.
The common voltage VCOM supplied to the gate driving unit 6 through the common voltage line 32 is applied to Ag dots (not shown) formed at edges of the thin film transistor array substrate. Since the Ag dot electrically connects the thin film transistor array substrate and the color filter substrate, the common voltage VCOM applied to the Ag dot is applied to the color filter substrate.
The control signals and driving voltages for driving the LCD device are supplied to the gate driving unit 6 from the timing control unit. Especially, a gate low voltage VGL and a gate high voltage VGH are applied to the gate driving unit 6. The gate low voltage VGL is applied to all the gate lines 16, and the gate high voltage VGH is sequentially applied to the gate lines 16 one by one, thereby turning-on the thin film transistor, the switching device provided at the pixels P1. At this time, image information is supplied to the pixels P1 through the data lines 15.
An electrostatic preventing unit 10 is respectively provided at one side of the data lines 15 and the gate lines 16. One side of the electrostatic preventing unit 10 is electrically connected to the data lines 15 or the gate lines 16, and another side of the electrostatic preventing unit 10 is connected to the common voltage line 32. The common voltage line 32 is electrically connected to the electrostatic preventing units 10 connected to the gate lines 16 and the electrostatic preventing units 10 connected to the data lines 15.
The electrostatic preventing units 10 are not operated at ordinary times. However, when an over-current is suddenly applied to the data lines 15 and the gate lines 16 due to an electrostatic inflow, the electrostatic preventing units 10 are conductive and thereby the gate lines 16 and the data lines 15 are connected through the common voltage line 32. This permits the current to be dispersed amongst the gate lines 16 and the data lines 15, thereby minimizing damage of the LCD device due to an electrostatic discharge.
The electrostatic preventing unit 10 protects each kind of device at the time of an instantaneous high voltage occurrence. The electrostatic preventing unit 10 has to be formed so that a current can not flow thereto at ordinary times. The gate low voltage VGL or the gate high voltage VGH outputted from the gate driving unit 6 is applied to one side of the electrostatic preventing unit 10, and the common voltage VCOM outputted from the common voltage line 32 is applied to another side of the electrostatic preventing unit 10. A voltage difference is thus generated between both sides of the electrostatic preventing unit 10. Therefore, a leakage current flows through the electrostatic preventing unit 10 due to the voltage difference even when the electrostatic preventing unit 10 is cut-off, and thereby a voltage level of the gate low voltage VGL or the gate high voltage VGH applied to each gate line 16 may be changed. When the voltage level of the gate low voltage VGL becomes unstable due to the leakage current, the pixels of the image display unit 20 cause a voltage change and thereby degrade the picture quality.